A detailed investigation of retroviruses in four new human breast cancer cell lines did not yield unequivocal evidence for viral functions. The cell lines did not display particle-associated RNA and RNA directed DNA polymerase. The genome of these cells did not contain mouse mammary tumor virus (type B) related DNA sequences. Similarly, electron microscopic search for budding particles gave negative results. In a related study with mouse model system, we found that mouse breast tumor cells (Mm5mt/c1) display age-related switches in retroviral gene expression. While the early passage cells produce mainly type B virus (MMTV), the late passage cells produce predominantly ecotropic type C virus. Though type B virus production is shut down, the late passage cell continues to synthesize type B viral RNA. The lesion appears to lie at the level of processing of type B viral mRNA.